Disk drives are capable of storing large amounts of digital data in a relatively small area. Disk drives store information on one or more recording media, which conventionally take the form of circular storage disks (e.g. media) having a plurality of concentric circular recording tracks. A typical disk drive has one or more disks for storing information. This information is written to and read from the disks using read/write heads mounted on actuator arms that are moved from track to track across the surfaces of the disks by an actuator mechanism.
Generally, the disks are mounted on a spindle that is turned by a spindle motor to pass the surfaces of the disks under the read/write heads. The spindle motor generally includes a shaft mounted on a base plate and a hub, to which the spindle is attached, having a sleeve into which the shaft is inserted. Permanent magnets attached to the hub interact with a stator winding on the base plate to rotate the hub relative to the shaft. In order to facilitate rotation, one or more bearings are usually disposed between the hub and the shaft.
Over the years, storage density has tended to increase, and the size of the storage system has tended to decrease. This trend has lead to greater precision and lower tolerance in the manufacturing and operating of magnetic storage disks. For example, to achieve increased storage densities, the read/write heads must be placed increasingly close to the surface of the storage disk. This proximity requires that the disk rotate substantially in a single plane. A slight wobble or run-out in disk rotation can cause the surface of the disk to contact the read/write heads. This is known as a “crash” and can damage the read/write heads and surface of the storage disk, resulting is loss of data.
From the foregoing discussion, it can be seen that the bearing assembly that supports the storage disk is of critical importance. One typical bearing assembly comprises ball bearings supported between a pair of races that allow a hub of a storage disk to rotate relative to a fixed member. However, ball bearing assemblies have many mechanical problems, such as wear, run-out and manufacturing difficulties. Moreover, resistance to operating shock and vibration is poor because of low damping.
One alternative bearing design is a hydrodynamic bearing. In a hydrodynamic bearing, a lubricating fluid such as air or liquid provides a bearing surface between a fixed member of the housing and a rotating member of the disk hub. In addition to air, typical lubricants include gas, oil, or other fluids. Hydrodynamic bearings spread the bearing surface over a large surface area, as opposed to a ball bearing assembly, which comprises a series of point interfaces. This is desirable because the increased bearing surface reduces wobble or run-out between the rotating and fixed members. Further, the use of fluid in the interface area imparts damping effects to the bearing, which helps to reduce non-repeat run-out.
Dynamic pressure-generating grooves (i.e. hydrodynamic grooves) disposed on journals, thrust, and conical hydrodynamic bearings generate a localized area of high fluid pressure and provide a transport mechanism for fluid or air so that fluid pressure is more evenly distributed within the bearing and between the rotating surfaces. The shape of the hydrodynamic grooves is dependent on the pressure uniformity desired. The quality of the fluid displacement and therefore the pressure uniformity is generally dependent upon the groove depth and dimensional uniformity. For example, a hydrodynamic groove having a non-uniform depth may lead to pressure differentials and subsequent premature hydrodynamic bearing or journal failure.
As the result of the above problems, electrochemical machining (ECM) of grooves in a hydrodynamic bearing has developed. Broadly described, ECM is a process of removing material metal without the use of mechanical or thermal energy. Basically, electrical energy is combined with a chemical to form an etching reaction to remove material from the hydrodynamic bearing, forming hydrodynamic grooves thereon. To perform the method, direct current is passed between the work piece, which serves as an anode, and the electrode, which typically carries the pattern to be formed and serves as a cathode. The current is passed through a conductive electrolyte that is between the two surfaces. At the anode surface, electrons are removed by current flow, and the metallic bonds of the molecular structure at the surface are broken. These atoms form a solution with the electrolyte, as metal ions, forming metallic hydroxides. These metallic hydroxide (MOH) molecules are carried away and filtered out from the electrolyte.
In current motor designs, “relief cuts” are machined into a work piece at one step in the machining process. These relief cuts have the effect of increasing the bearing running gap in certain areas, hence creating less friction loss by unnecessary shearing of oil. Therefore, power consumed by the bearings is reduced. However, this additional step in the machining process renders the overall process longer and therefore more costly.
Therefore, a need exists for an electrochemical machining process that reduces bearing power consumption without requiring additional cost or time during manufacturing.